This invention relates to a novel method and apparatus for offshore drilling operations. More specifically, this invention relates to a dual riser method and apparatus for use in drilling and/or production work over of a single well hole in deep water applications. The subject invention enables a deep water drilling rig, having dual turntables, to work simultaneously through two parallel risers to shorten the critical path associated with deep water drilling and/or work over activity.
Significant oil and gas reserves have been discovered beneath various bodies of water throughout the world. Originally, the state of technology limited offshore drilling and production to relatively shallow locations in shoreline areas where the depth of the water ranged from a few feet to several hundred feet. The extensive exploration and removal of resources from these near shore regions, coupled with a constant demand for cost effective energy from large, productive reserves, have led to a search for and drilling of oil and gas reserves in locations beneath greater depths of water.
Presently, the industry is conducting drilling operations in depths of 7,500 feet of water, and it is anticipated that these operations will migrate to even deeper waters since the industry has begun leasing blocks for drilling in areas where the depth of water can be ten thousand feet or more. In this, it is predicted that the oil industry will soon be drilling in depths of water twelve thousand feet or more. These desires will only grow as technology, such as seismic imaging, continues to progress and identify locations of substantial oil and gas reserves that are buried under even greater depths of water.
In the past, shallow-water offshore drilling operations have been conducted from fixed towers and mobile units, such as jack-up platforms. These units are usually assembled on shore and then transported to an offshore drilling site. For a tower unit, the towers are erected over a proposed well head and fixed to the marine floor. A jack-up platform may be transported to the site through the use a barge or through a self-propulsion mechanism on the platform itself. Once the platform is over the proper location, legs on the corners of the barge or self-propelled deck are jacked down into the seabed until the deck is positioned above the statistical storm wave height. These jack-up barges and platforms drill through a relatively short riser in a manner similar to land based operations. Although jack-up rigs and fixed platforms work well in depths of water that total approximately a few hundred feet, they do not work well in deep water operations.
For operations in deep water, semi-submersible platforms have been successfully utilized, such as disclosed in United States Ray et al. U.S. Pat. No. 3,919,957 and Steddum U.S. Pat. No. 3,982,492. Tension leg platforms are designed with a platform and a plurality of cylindrical legs or columns which are buoyant and extend into the sea. Tension leg platforms are held in place by anchors that are fixed into the seabed and by a plurality of permanent mooring lines connected beneath each buoyancy column. These mooring lines are tensioned to counteract the buoyancy of the legs and stabilize the platform. A further example of a tension legged platform is disclosed in United States Ray et al. U.S. Pat. No. 4,281,613.
For sites with even greater depths of water, turret moored drillships and dynamically positioned drillships serve as a platform for drilling operations. Turret moored drillships are depicted in United States Richardson et al. U.S. Pat. Nos. 3,191,201 and 3,279,404. Dynamically positioned drillships are similar to turret moored drillships in that drilling occurs through a large central opening, or moon pool, fashioned vertically through the vessel amid ships. Bow and stern thruster sets cooperate with multiple sensors and computer controls to maintain the vessel at set coordinates. A dynamically controlled drillship and riser angle positioning system is disclosed in United States Dean U.S. Pat. No. 4,317,174.
Regardless of the equipment utilized, whenever drilling operations are conducted in deep water, greater costs are encountered as compared to operations in shallower depths of water. These increased costs are compounded by the additional time needed to make-up and break down drillstrings during a conventional drilling operation.
In a conventional offshore drilling operation thirty inch (30xe2x80x3) casing is first jetted into the initial mudline of a well hole and is cemented into position. A twenty-six inch (26xe2x80x3) hole section is then drilled through the casing. The twenty-six inch (26xe2x80x3) drilling assembly is then pulled back to the surface and twenty inch (20xe2x80x3) tubular casing is landed on the well head and the twenty inch (20xe2x80x3) casing is cemented into place. An eighteen and three-quarters inch (18xc2xexe2x80x3) blowout preventer (xe2x80x9cBOPxe2x80x9d) stack is connected to the bottom of a twenty-one inch (21xe2x80x3) riser and lowered onto the well head and tested. After this operation is completed and the twenty-one inch (21xe2x80x3) riser is set, all further drilling actually takes place through the single twenty-one inch (21xe2x80x3) riser. This includes drilling a seventeen and one-half inch (17xc2xdxe2x80x3) hole, running and cementing thirteen and three-eighths inch (13xe2x85x9cxe2x80x3) casing, drilling a twelve and one-quarter inch (12xc2xc) hole section, running and cementing a nine and rive-eighths inch (9⅝xe2x80x3) casing, drilling an eight and one-half inch (8xc2xdxe2x80x3) hole, etc.
Each segment of the drilling operation including changing bits requires casing or drill pipe segments to be made up in thirty-one foot (31xe2x80x3) segments at a rotary drillship station and lowered to the seabed in increments.
Drilling time was significantly reduced in offshore operations by the development of a dual-activity drillship by Scott et al. in the above-referenced U.S. Pat. No. 6,081,851 entitled xe2x80x9cMulti-Activity Offshore Exploration and/or Development Drilling Method and Apparatus.xe2x80x9d The disclosure of this Scott et al. patent is hereby incorporated by reference as though set forth at length.
Notwithstanding the significant advances provided by the Scott et al. dual-activity drillship invention, once the BOP stack was mounted at the bottom of the twenty-one inch (21xe2x80x3) riser and latched to the well head all further drilling activity must be conducted through the riser.
In addition to thousands of feet drilled into the seabed, for an operation conducted at a water depth of 7,500 feet, the extra time taken to cycle any drilling assembly through the drilling riser from the drillship to seabed averages about five hours per cycle. Since the design of a normal rig only provides for drilling through one rotary table onto which the single drilling riser is attached, drilling operations must be ceased during the period of time needed to pull a spent drilling assembly from a well up the riser and also during the period while a new drilling assembly is lowered down the riser and into the well.
Accordingly, it would be desirable to increase the drilling efficiency of a dual-activity drillship, even further, by reducing lost time drawing up and paying out drillstrings through the drilling riser running from the drillship to the seabed in deep water.
Objects
It is, therefore, a general object of the subject invention to provide a novel, deep water, drilling method and apparatus operable to enhance the drilling efficiency of a dual-activity drilling assembly.
It is a specific object of the invention to provide a novel method and apparatus for reducing the time involved in drilling wells located beneath substantial depths of water.
It is a particular object of the subject invention to reduce the operational time needed for cycling drilling assemblies through a marine riser section of deep water drilling activity.
It is another object of the subject invention to allow a multi-activity drilling assembly to operate efficiently in locations where the water depths of seven thousand feet or more.
It is a further object of the invention to provide a novel method and apparatus removing significant segments of time from the critical path of a deep water drilling operation.
It is a related object of the invention to provide a novel method and apparatus of enhanced activity to fully utilize the capacity of a dual-activity drillship of the type described in U.S. Pat. No. 6,085,851.
It is a specific object of the invention to provide a novel dual riser, deep water, drilling method and apparatus operable for permitting two drilling and/or casing string to be run simultaneously from a drillship to a well hole in a posture operable for selective insertion into a subsea well hole.
A preferred embodiment of the invention which is intended to accomplish at least the above-referenced objects comprises a dual riser assembly for use with an offshore multi-activity drilling assembly having provision for a pair of risers. The invention is designed to conduct drilling procedures between the deck of a dual-activity drilling assembly above the surface of the body of water and a single well location in the bed of the body of water.
The dual riser assembly is operable to be connected to a single BOP of a well hole and includes plural riser segments. A first riser segment has a longitudinal axis substantially coincident with the longitudinal axes of a first riser from the surface drilling assembly and the well hole. A second riser segment extends from the dual riser assembly at an acute angle with respect to the first riser segment and is in selective communication with the first riser segment.
Each riser segment of the subject invention is equipped with a valve, or blind rams, that may be independently opened or closed to respectively connect or seal off the riser above the well hole. The isolating properties of these valves accommodate the method of running simultaneous drillstrings in a non-active riser to a point above the valves without disrupting any activity being performed through the corpus of the assembly and well hole from the active riser.
In one embodiment of the invention a flex joint is positioned between the base of the dual riser assembly and the head of a BOP stack such that an active one of two marine risers may be brought into axial alignment with the bore of the well hole and eliminate any tendency for alignment wear at the junction between the dual riser assembly and the BOP stack.